Characterisation of water transfer in a low temperature convective wood drier: influence of the operating parameters on the mass transfer coefficient

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Characterisation of water transfer in a low temperature convective wood drier: influence of the operating

parameters on the mass transfer coefficient

L. Chrusciel, E Mougel, A Zoulalian, T. Meunier

To cite this version:

L. Chrusciel, E Mougel, A Zoulalian, T. Meunier. Characterisation of water transfer in a low temper- ature convective wood drier: influence of the operating parameters on the mass transfer coefficient.

Holz als Roh - und Werkstoff, 1999, �10.1007/s001070050070�. �hal-01765065�

Characterisation of water transfer in a low temperature convective wood drier: influence of the operating parameters

on the mass transfer coefficient

L. Chrusciel, E. Mougel, A. Zoulalian, T. Meunier

Different drying cycles were performed in a low temper- ature convective and homogeneous wood drier in order to evaluate the in¯uence of four operating parameters (wood thickness, air velocity, air temperature and air humidity) on the global mass transfer coef®cient. The wood species used in this study was spruce (Picea abies Karst.). From the synthesis of the results, a general correlation has been obtained permitting to accurately calculate this transfer coef®cient. Moreover, it justi®ed the use of the simple mathematical model chosen to represent the variations of the parameters characterising the wood and air during a drying cycle.

Charakterisierung des Wasserflusses in einem Niedrigtemperatur-Trockner: Einfluû der Variablen auf den Koeffizienten fuÈr Massentransport In der Absicht, den Ein¯uû von vier Variablen (Holz- dichte, Luftgeschwindigkeit, Lufttemperatur und Luft- feuchte) auf den Koef®zienten fuÈr Massentransport zu bestimmen, wurden verschiedene Trocknungszyklen in einem homogenen Niedrigtemperatur-Trockner durch- gefuÈhrt. Die hierfuÈr verwendete Holzart war Fichte (Picea abies Karst). Im Anschluû an die Zusammenfassung der Ergebnisse wurde eine allgemeine Korrelation, mit der dieser Koef®zient auf eine bestimmte Weise berechnet werden kann, erzielt. Diese rechtfertigte die Wahl eines einfachen Simulationsmodells, das die VeraÈnderung der charakteristischen Luft- und Holz- parameter waÈhrend der Trocknung darstellt.

List of symbols A Constant a

Constant B Constant b

Constant c

Constant

Cp Speci®c heat, J kg

K

D Diffusion coef®cient of water in wood, m

s

e Wood thickness, mm

G Mass ¯ow rate of dry air, kg s

h Heat transfer coef®cient, w m

K

i Iterative variable

k

Partial mass transfer coef®cient in the gas phase, kg m

s

k

Partial mass transfer coef®cient in the solid phase, kg m

s

k

Global mass transfer coef®cient, kg m

s

M Mass of wood, kg

m Repartition coef®cient, /

m

Final mass of a piece of wood, kg M

Oven dry mass of wood, kg

N Number of wood pieces in the drier, / p Constant

Rh Air relative humidity, %

S Exchange surface between wood and air, m

T Air temperature, K;

C

t Wood temperature, K;

C v Air velocity, m s

w Air moisture content, kg kg

x

Final moisture content of a piece of wood, kg kg

x

Wood moisture content at the ®bre saturation point,

kg kg

x Wood moisture content, kg kg

x

Equilibrium moisture content, kg kg

z Residual air desiccation ratio, /

Greek letters a Constant

K Latent heat of water in wood at the temperature t, J kg

K

Latent heat of water in air at T ˆ 0

C, J kg

s Time, s

Subscripts a Air

b Wood

C Core I Drier input O Drier output 0 Initial time S Surface w Water Exponents L Liquid phase V Vapour phase

1 Introduction

In the wood industry, the control of a drying cycle requires knowing the variations of the wood moisture content. It is

Holz als Roh- und Werkstoff 57 (1999) 439±445ÓSpringer-Verlag 1999

Originalarbeiten á Originals

L. Chrusciel, E. Mougel, A. Zoulalian ENSTIB, LERMAB, Univ. H. PoincareÂ

Nancy 1, BP 1041, F-88051 Epinal cedex 9, France T. Meunier

EDF ± DER deÂpartement systeÁmes eÂnergeÂtiques Centre de Recherches Les RenardieÁres ± BP 1, France

439

very common to follow these variations by electrical nee- dle electrodes method. As speci®ed by the US Forest Products Laboratory (1974) or by Joly and More-Chevalier (1980), experience has shown that these probes do not permit to obtain accurate values of the average wood moisture content during drying. Moreover, the measure- ments by needle electrodes are limited in the range 6±30%.

The major drawback of these probes is relative to their operating mode. Since wood is a heterogeneous material, therefore wood moisture content measurements have only local signi®cance. Consequently, a great number of these probes are required to measure satisfactorily the average wood moisture content. From a technical point of view, this approach is unreasonable and the kiln operators in the wood industry ciecumvent this problem by empirical knowledge (they often increase the duration of a drying cycle in order to be sure that the ®nal wood moisture content has really been obtained) and by measuring the difference between the input and output air temperature.

Because it is dif®cult to obtain accurate values of wood moisture content without damage, it can be calculated. In the past, investigations concerning the modelling of wood drying have been made. To describe the transfer of water vapour and of bound water, Fick's law is often used, which requires knowledge of the water diffusion coef®cient. The work of Hernandez and Puiggali (1994) modelling the drying of a hygroscopic capillary-porous media with ap- plication to softwood, has to be mentioned. Various pro- cesses are simulated, and in particular convective drying at different temperatures below 100

C. The study concerning lumber drying presented by Bramhall (1979) is very complete: all the physical parameters are considered and expressed by mathematical relations. The author, as well as later Simpson (1991), presents the coef®cient of diffu- sion as a function of wood moisture content rather than integrating the in¯uence of the temperature as Stamm (1964). Many empirical correlations permitting to calcu- late this coef®cient of diffusion exist. But as underlined by Moschler and Martin (1968), the value of this coef®cient strongly depends on the method developed to measure it.

Consequently, the use of these correlations seems to be very restrictive. Another way to model wood drying is to introduce a convective mass transfer coef®cient like Gui et al. (1994). The model developed by the authors includes the non-homogeneous properties of wood. At present, the tendency in the research is to add equations in order to improve the accuracy of modelling and to take the orthotropicity of wood material into account. It must be underlined that the representations of mass transfer dur- ing wood drying are thereby very sophisticated from a conceptual point of view and have too strong a depen- dence on the values of parameters introduced in the model and describing transfer.

These are the reasons why a simple model was developed in order to represent heat and mass transfers characterised by two global coef®cients. It was shown in a previous study (Karabagli et al., 1997) that the working of a low temper- ature convective and homogeneous wood drier can be satisfactorily represented by a model in which the wood and air moisture contents verify the system of equations resulting from: the mass balance of water in drying air

G … w

ÿ w

† ˆ k

S … x ÿ x

† …1†

the enthalpy balance on drying air G ‰ Cp

… T

ÿ T

† ‡ K

… w

ÿ w

†

‡ Cp

… w

T

ÿ w

T

†

ˆ k

S K … x ÿ x

† ÿ h S … T

ÿ t

† …2†

the mass balance of water in wood ÿM

dx

ds ˆ k

S … x ÿ x

† …3†

the enthalpy balance of wood M

Cp

‡ x Cp

dt

ˆ ÿG ‰ Cp

… T

ÿ ds T

† ‡ … w

ÿ w

† ÿK ÿ

ÿ Cp

t

‡ Cp

… w

T

ÿ w

T

† …4†

In the equations above, the transfer of water between wood and air is characterised by the mass transfer coef®cient k

, considered as a constant during drying. The different drying cycles studied in the past have shown that wood species do not have a signi®cant in¯uence on the value of the coef®cient k

. In order to be able to use the model presented above in many different conditions of drying, the in¯uence of the main operating parameters (charac- terising drying) on the mass transfer coef®cient is inves- tigated ®rst. A general correlation obtained from the synthesis of all the results is then proposed to allow the evaluation of the global mass transfer coef®cient in func- tion of the operating parameters considered.

2 Materials and methods

2.1 Experimental apparatus

The experimental work of this study was carried out on a wood drier which is similar, considering its process, to industrial driers: the major difference concerns its size and its probes equipment. Indeed, the average capacity of industrial driers is some ten cubic meters versus only 0.4 cubic meter for the drying cell used in this study (see scheme, Fig. 1).

For drying, the wood is stacked in the drying cell S crossed by an air ¯ow whose input humidity and tem- perature are regulated. A vapour generator G and an electric resistance R are situated on the main circuit of the equipment. A condenser C is positioned on the dehydra- tion circuit. The function of these three devices is to in- crease or to decrease the temperature and humidity of the drying air. The air ¯ow in the main circuit is blown by a ventilator V. Depending on the opening of the two sluices Vp and Vd, the rate of the drying air ¯ow can vary between 600 and 2000 m

=h.

Several variables were measured during a drying cycle:

the temperature, relative humidity and the rate of the air

¯ow in both the main circuit and dehydration circuit, and

the mass of wood. Three thermocouples are disposed at

the points 1, 2 and 3 of the drying plant (see Fig. 1), two

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